Recently, as technology development and demand for mobile devices has been increasing, the demand for rechargeable batteries capable of being charged and discharged as an energy source has been rapidly increasing, and accordingly, a great deal of research on rechargeable batteries that can meet various demands has been conducted. Also, the rechargeable batteries are attracting attention as a power source for electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (Plug-In HEV), etc. that are presented as a solution to solve air pollution and the like of conventional gasoline vehicles and diesel vehicles using fossil fuels. Therefore, the electric vehicles (EV) that can operate only with the battery, as well as the hybrid electric vehicles (HEV) using the battery and the conventional engine, are being developed, and some are commercially available. The rechargeable battery as the power source such as for the EV, the HEV, and the like is generally a nickel metal hydrogen (Ni-MH) rechargeable battery, and recently, research on lithium secondary batteries with high energy density, high discharge voltage, and good output stability have been actively conducted, and several are in commercialization stages. When the rechargeable battery is used as the power source of the vehicle, the rechargeable battery is used in a form of a battery pack including a plurality of battery modules in battery module assemblies. In this case, to use the battery pack as the power source of a device or a system that is exposed to various environments such as the vehicle, it must be able to maintain structural stability with respect to stimuli such as external impacts depending on various environments and ensure the safety of the battery pack at varying humidity and temperature. Accordingly, several battery packs used as the power source of the vehicles are mounted in an internal space like a part of a seat area from the trunk of the vehicle to easily secure a mounting space and reduce the stimuli due to various factors such as the external impact and humidity. However, these vehicle battery packs occupy a large portion of the internal space due to their large volume, and there are problems that the trunk space may not be fully utilized and the seat of the vehicle must be reduced. Therefore, there is a high necessity of a technology capable of fundamentally solving such problems. The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.